This invention relates to optical fibers and, more particularly, to an optical fiber having an inner cladding for receiving pump radiation that is to be absorbed by active material in the core of the optical fiber.
Optical fiber lasers and amplifiers are known in the art. In such lasers and amplifiers, rare earth materials disposed in the core of the optical fiber laser or amplifier absorb pump radiation of a predetermined wavelength and, responsive thereto, provide or amplify light of a different wavelength for propagation in the core. For example, the well-known erbium doped fiber amplifier (EDFA) receives pump radiation having a wavelength of 980 or 1480 nanometers (nm) and amplifies an optical signal having a wavelength in the 1550 nm region and that propagates in the core.
In such optical fiber lasers and amplifiers, the pump radiation can be introduced directly to the core, which can be difficult due to the small size of the core, or can be introduced to the cladding layer surrounding the core and absorbed by the core as the rays propagating in the cladding layer intersect the core. Lasers and amplifiers with the pump radiation introduced to the cladding layer are known as xe2x80x9cdouble-cladxe2x80x9d or xe2x80x9ccladding-pumpedxe2x80x9d optical devices, and facilitate the scale-up of lasers and amplifiers to higher power systems.
FIG. 1 illustrates an optical fiber having a core 20, an inner, or pump, multimode cladding layer 22, and an outer cladding layer 24. The inner cladding layer 22 confines light rays 26, which represent the light generated or amplified in the core 20, to the core 20. Similarly, the outer cladding 24 confines light rays 28, which represent pump radiation propagating in the inner cladding 22, to the inner cladding 22. Note that the rays 28 periodically intersect the core 20 for absorption by the active material therein so as to generate or amplify the light propagating in the core 20, represented by the rays 26. Because the inner cladding 22 is multimode, many rays other than those shown by reference numeral 28 can propagate in the inner cladding 22.
Absorption per unit length is a useful figure of merit for evaluating a double-clad optical fiber laser or amplifier. It is typically desirable that the amplifier or laser has a high absorption per unit length, indicating that the pump radiation frequently intersects the core. It has been determined by various researchers over the years that a standard circular fiber geometry, such as is desirable when fabricating an optical fiber for transmission over substantial distances, does not optimally promote intersection of the core by the radiation pumped into the cladding. Unfortunately, some rays of the pump radiation, known in the art as xe2x80x9cskewxe2x80x9d rays, can essentially propagate down the optical fiber while spiraling around the core without substantially intersecting the core, as shown in FIG. 1B, where pump radiation rays 28A do not intersect the core 20. The existence of skew rays leads to a low absorption per unit length of the optical fiber device, and hence detracts from the performance of the optical fiber laser or amplifier.
The prior art teaches two approaches for enhancing the intersection of the pump radiation with the core and hence raising the absorption per unit length of the optical fiber amplifier or laser. In the first approach, the core is relocated to intersect more of the rays of the pump radiation. For example, as shown in FIG. 2A and disclosed in U.S. Pat. No. 4,815,079, issued Mar. 21, 1989 to Snitzer et al., the core can be offset from the center of the optical fiber so as to enhance the intersection of pump light with the core.
In the second approach, the shape of the outer circumference of the inner, or pump, cladding layer is modified to xe2x80x9cmode mixxe2x80x9d or scatter more rays towards the core so as to intersect with the core. For example, as shown in FIG. 2B and also disclosed in the xe2x80x2079 patent to Snitzer, the inner cladding can have a rectangular outer circumference. See also FIG. 2C, where the inner cladding has a xe2x80x9cDxe2x80x9d-shaped outer circumference that includes a flat section, as disclosed in U.S. Pat. No. 5,864,645, issued Jan. 26, 1999 to Zellmer et al. In yet another example of this approach, the outer circumference of the cladding is shaped as a polygon, such as a hexagon, as disclosed in U.S. Pat. No. 5,533,163, issued Jul. 2, 1996 to Muendel and shown in FIG. 2D. In yet further examples, the outer circumference of the inner cladding has a xe2x80x9cstarxe2x80x9d shape, as disclosed in U.S. Pat. No. 5,949,941, issued Sep. 7, 1999 to DiGiovanni and illustrated in FIG. 2E. See also WO 99/30391, published Jun. 17, 1999, disclosing an optical fiber having a core, inner and outer claddings, and a series of circularly shaped perturbations or irregularities formed in the otherwise circular outer boundary of the inner cladding, as shown in FIG. 2F. The optical fiber is drawn from a preform having rods inserted into holes drilled into the preform.
The prior art approaches discussed above can have disadvantages. For example, the fibers can be difficult to splice to a fiber having a standard, circular geometry in a manner that provides for an acceptably low loss of light, as is often required in a practical application. The offset core fiber of FIG. 2A can be particularly difficult to splice. Furthermore, designs shown in FIGS. 2B-2F, wherein the outer circumference of the inner cladding is shaped, can require shaping of the preform from which the fiber is drawn. Shapes that include flat areas, such as the polygon design discussed above, can be difficult and/or time consuming, and hence more expensive, to fabricate. The flat areas are typically first machined into the preform from which the optical fiber is drawn. Furthermore, shaped areas of the preform to deform and change shape when the fiber is drawn at the most desirable temperatures. Accordingly, often the draw temperature is reduced to preserve the desired shape of the outer circumference of the cladding. A reduced draw temperature typically produces optical fibers having higher attenuation and lower mechanical strength.
Accordingly, although the approaches described above may represent an improvement in the art, a double-clad fiber that addresses one or more of the foregoing disadvantages and drawbacks of the prior art approaches would be a welcome advance in the art.
In one aspect, the present invention provides a double-clad optical fiber that includes the following: a core having a first index of refraction and including an active material; a multimode inner cladding layer for receiving pump radiation, the inner cladding layer disposed about the core and including material having a second index of refraction that is less than the first index of refraction; a second cladding layer disposed about the inner cladding layer, the second cladding layer having a third index of refraction that is less than the second index of refraction. The multimode inner cladding of the double-clad fiber includes truncated regions having an index of refraction that is different than the material of the inner cladding that surrounds the truncated regions. Accordingly, the truncated regions promote the scattering of pump radiation propagating in the multimode inner cladding for increasing the absorption by the core of pump radiation.
In another aspect of the invention, the truncated regions can include filaments extending along the length of the double-clad fiber. The optical fiber can be drawn from a preform wherein the inner cladding is formed at least in part via outside vapor deposition, and wherein particles of the first material are distributed with material of the inner cladding deposited via outside vapor deposition. Alternatively or additionally, the double-clad optical fiber can be drawn from a preform formed at least in part from a frit, and wherein the first material includes material introduced at least in part by exposure of the frit to a selected solution. The truncated regions can also include voids defined by material of the inner cladding, and which may be filled with a gas.
In yet a further aspect of the invention, the truncated regions are concentrated nearer to the outer circumference of the inner cladding than to the core of the optical fiber. The truncated regions can be distributed in a band spaced from the core of the fiber by a region of the inner cladding having substantially no truncated regions. The distribution of truncated regions can include truncated regions having a maximum diameter of less than 100 microns. The active material can includes at least one of erbium, ytterbium, neodymium and thulium and other rare earth materials.
The double-clad optical fiber can include at least one bend. Bending the fiber is considered to promote mode mixing of the light in the inner cladding and hence a higher absorption of the pump radiation by the active material per unit length of the double-clad optical fiber.
The invention also includes methods practiced in accordance with the teachings herein.
In one aspect, a method of forming a cladding for being disposed about the core of an elongate optical article can include the following steps: providing a elongate glass article; adding glass to the article for forming a first part of the cladding so as to disposed about the core when present, the added glass including discrete regions having a different index of refraction than the added glass; and adding glass without discrete regions to the elongate glass article for forming another part of the same cladding so as to be disposed about the core when present.
In an additional aspect of the invention, a method of forming a cladding for being disposed about the core of an optical article can include the following steps: providing an elongate glass article; adhering a layer of soot to the elongate glass article for forming a portion of the cladding so as to be disposed about the core when present; sintering the layer of soot so as to form a first sintered layer including bubbles; adhering a different layer of soot to the elongate glass article for forming a different portion of the cladding so to be disposed about the core when present; sintering the different layer of soot so as to form a different sintered layer substantially free from bubbles; and disposing a second cladding about the cladding, where the second cladding has an index of refraction lower than that of the cladding.
In another aspect of the invention, a method of forming a cladding for being disposed about the core of an optical article can include the following steps: providing a hollow elongate glass article; adhering a layer of soot to the elongate glass article for forming a portion of the cladding so as to be disposed about the core when present; sintering the layer of soot so as to form a sintered layer including bubbles; providing a second elongate glass article for providing one of at least a portion of the core and a different portion of the cladding where the different portion is substantially free of bubbles; and oversleeving one the glass articles with the other of the glass articles.
In a further aspect of the invention, a method of forming a cladding for surrounding the core of an optical article can include the following steps: providing a elongate glass article; adhering a layer of soot to the elongate glass article for forming a portion of the cladding so as to be disposed about the core when present; sintering said layer of soot so as to form a first sintered layer of the cladding; adhering a different layer of soot to the elongate glass article for forming a different portion of said cladding so as to be disposed about the core when present; exposing only the different layer of soot to a selected material in the form of a gas or liquid for absorption by the different layer of soot; and sintering the different layer of soot so as to form a second sintered layer of said cladding.
In yet another aspect of the invention, a method of forming a cladding for being disposed about the core of an optical article can include the following steps: providing a elongate glass article; adhering a layer of soot to the elongate glass article for forming a portion of the cladding so as to be disposed about the core when present; distributing particles having an index of refraction different than the index of refraction of the soot with the layer of soot; and sintering the soot layer.
In yet another additional aspect of the invention, a method of forming a cladding for being disposed about the core of an optical article can include the following steps: providing a hollow elongate glass article; adhering a layer of soot to the inside of the elongate glass article for forming a portion of the cladding so as to be disposed about the core when present; exposing the layer of soot to a selected material in one of a gas and liquid form for absorption by the soot; sintering the soot; providing a second glass article for providing one of at least a portion of the core and a different portion of the cladding; and oversleeving one of the glass articles with the other of the glass articles.
Thus the invention can provide a double-clad optical fiber that promotes absorption by active material in the core of pump radiation and that can retain, if desired, a circularly shaped inner cladding for improved splicing to standard optical fibers.